This invention relates to synchronous motors and, particularly, to stepping motors.
Stepping motors and their operation are well known. In an article "New Inductor Motor Has Low Speed, Self-Start and High Torque" by J. H. Staak in the June, 1947 of Electrical Manufacturing, Volume 39, No. 6, published by Gage Publishing Company, the author describes an early stepping motor. In such a motor, a rotor composed of two axially displaced sets of permanently-magnetized radially-extending rotor teeth rotates about an axis common to a stator composed of inwardly extending stator poles, each having stator teeth. One set of rotor teeth is peripherally displaced relative to the other set by one-half tooth pitch. The stator teeth extend axially across both sets off rotor teeth and actuate the rotor by virtue of electrical signals such as pulses applied to windings surrounding the individual stator poles. Typically, in such motors, the two sets of rotor teeth form two soft iron pole shoes on either side of an axially magnetized permanent magnet.
One of the advantages of a stepping motor is its ability to rotate in synchronism with an applied voltage. It is able to "step" or rotate the rotor through a predetermined stepping angle in response to an input voltage or current pulse. In general, the stepping angle, or the angle of a step, through which a rotor responds to a single pulse is determined by the number of rotor teeth, the number of stator poles, and the manner in which the windings are pulsed. Another advantage of a stepping motor is that it is possible to provide for cogging during rotation or for smooth rotation, depending upon the desired application, by selecting a proper tooth pitch relative to the rotor tooth pitch and by proper choice of voltages and currents applied to the windings.
The manner in which pulsing of the windings varies the stepping angle through which the motor rotates the stator is described in U.S. Pat. No. 4,225,696. This patent stows how the rotor may be rotated through full steps, half steps, quarter steps, and eighth steps. A full step corresponds movement of the rotor one-quarter tooth pitch in an eight pole motor.
Stepping motors have a number of disadvantages. To achieve maximum efficiency, the two sets of rotor teeth must be peripherally offset from each other by one-half rotor tooth pitch with as much precision as possible. Aligning the sets to satisfy this requirement complicates the manufacture and assembly of stepping motors. Improper alignment of the sets, and lack of alignment uniformity from rotor to rotor, causes variations in the torque displacement characteristics. This is particularly so during half-stepping or microstepping (stepping at less than one-half step) the rotor. These variations are undesirable where a large number of motors must operate in a large number of applications in response to identical inputs to achieve identical outputs. An example of such use is in floppy disc drives.
Furthermore, stepping motors tend to exhibit resonances. Stepping motors also tend to vibrate.
An object of this invention is to improve stepping motors.
Another object of this invention is to alleviate the aforementioned problems.